I have a FAT32 part. with the dtb and uImage file. This seems to load fine with fat load.

There is a aarch64 ARM compiled Root File System on a ext4 part. Also on the same USB.

Below is the UART output:

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BL2 Built : 14:59:36, Aug  9 2017.
gxl ge8c6a83 - xiaobo.gu@droid12

set vcck to 1120 mv
set vddee to 1000 mv
Board ID = 2
CPU clk: 1200MHz
DQS-corr enabled
DDR scramble enabled
DDR3 chl: Rank0+1 @ 912MHz - FAIL
DDR3 chl: Rank0 @ 912MHz - FAIL
DDR3 chl: Rank0 16bit @ 912MHz - FAIL
DDR4 chl: Rank0+1 @ 1008MHz - FAIL
DDR4 chl: Rank0 @ 1008MHz - PASS
Rank0: 1024MB(auto)-2T-18
DataBus test pass!
AddrBus test pass!
-s
Load fip header from eMMC, src: 0x0000c200, des: 0x01400000, size: 0x00004000
New fip structure!
Load bl30 from eMMC, src: 0x00010200, des: 0x01100000, size: 0x0000d600
Load bl31 from eMMC, src: 0x00020200, des: 0x05100000, size: 0x0002c600
Load bl33 from eMMC, src: 0x00050200, des: 0x01000000, size: 0x00066000
NOTICE:  BL3-1: v1.0(release):801e730
NOTICE:  BL3-1: Built : 22:09:38, Aug 10 2017
[BL31]: GXL CPU setup!
NOTICE:  BL31: BL33 decompress pass
mpu_config_enable:ok

[Image: gxl_v1.1.3244-a3237e2 2017-08-16 14:01:18 hong.guo@droid04]

OPS=0x82

eb 45 d f6 96 ed 83 f7 9b bd e0 79 [0.640225 Inits done]

secure task start!
high task start!
low task start!
ERROR:   Error initializing runtime service opteed_fast


U-Boot 2015.01 (Sep 15 2018 - 19:26:47)

DRAM:  1 GiB
Relocation Offset is: 36eb2000
register usb cfg[0][1] = 0000000037f59de0
[CANVAS]canvas init
vpu: error: vpu: check dts: FDT_ERR_BADMAGIC, load default parameters
vpu: clk_level = 7
vpu: set clk: 666667000Hz, readback: 666660000Hz(0x300)
vpu: vpu_clk_gate_init_off
vpp: vpp_init
boot_device_flag : 1
Nand PHY Ver:1.01.001.0006 (c) 2013 Amlogic Inc.
init bus_cycle=6, bus_timing=7, system=5.0ns
reset failed
get_chip_type and ret:fffffffe
get_chip_type and ret:fffffffe
chip detect failed and ret:fffffffe
nandphy_init failed and ret=0xfffffff1
MMC:   aml_priv->desc_buf = 0x0000000033eb26b0
aml_priv->desc_buf = 0x0000000033eb49d0
SDIO Port B: 0, SDIO Port C: 1
emmc/sd response timeout, cmd8, status=0x1ff2800
emmc/sd response timeout, cmd55, status=0x1ff2800
init_part() 293: PART_TYPE_AML
[mmc_init] mmc init success
      Amlogic multi-dtb tool
      Single dtb detected
start dts,buffer=0000000033eb7200,dt_addr=0000000033eb7200
      Amlogic multi-dtb tool
      Single dtb detected
parts: 12
00:      logo	0000000002000000 1
01:  recovery	0000000002000000 1
02:       rsv	0000000000800000 1
03:       tee	0000000000800000 1
04:     crypt	0000000002000000 1
05:      misc	0000000002000000 1
06:      boot	0000000002000000 1
07:    system	0000000080000000 1
08:     cache	0000000020000000 2
09:    vendor	0000000010000000 1
10:       odm	0000000010000000 1
11:      data	ffffffffffffffff 4
init_part() 293: PART_TYPE_AML
eMMC/TSD partition table have been checked OK!
check pattern success
mmc env offset: 0x27400000
In:    serial
Out:   serial
Err:   serial
reboot_mode=cold_boot
hpd_state=0
cvbs performance type = 6, table = 0
[store]To run cmd[emmc dtb_read 0x1000000 0x40000]
_verify_dtb_checksum()-917: calc 7293f130, store 7293f130
_verify_dtb_checksum()-917: calc 7293f130, store 7293f130
dtb_read()-1039: total valid 2
dtb_read()-1106: do nothing
      Amlogic multi-dtb tool
      Single dtb detected
Net:   dwmac.c9410000amlkey_init() enter!
[EFUSE_MSG]keynum is 4
[BL31]: tee size: 0

Start read misc partition datas!
info->attemp_times = 0
info->active_slot = 0
info->slot_info[0].bootable = 1
info->slot_info[0].online = 1
info->slot_info[1].bootable = 0
info->slot_info[1].online = 0
info->attemp_times = 0
attemp_times = 0
active slot = 0
wipe_data=successful
wipe_cache=successful
upgrade_step=2
[OSD]load fb addr from dts
[OSD]fb_addr for logo: 0x7e000000
[OSD]load fb addr from dts
[OSD]fb_addr for logo: 0x7e000000
[CANVAS]addr=0x7e000000 width=3840, height=2160
amlkey_init() enter!
amlkey_init() 71: already init!
[EFUSE_MSG]keynum is 4
[BL31]: tee size: 0
[KM]Error:f[key_manage_query_size]L507:key[usid] not programed yet
[KM]Error:f[key_manage_query_size]L507:key[mac] not programed yet
[KM]Error:f[key_manage_query_size]L507:key[deviceid] not programed yet
gpio: pin GPIOAO_2 (gpio 102) value is 1
Hit Enter or space or Ctrl+C key to stop autoboot -- :  1  0
gxl_p212_v1#

gxl_p212_v1#usb start

(Re)start USB...
USB0:   USB3.0 XHCI init start
Register 2000140 NbrPorts 2
Starting the controller
USB XHCI 1.00
scanning bus 0 for devices... 2 USB Device(s) found
       scanning usb for storage devices... init_part() 278: PART_TYPE_DOS
1 Storage Device(s) found
gxl_p212_v1#fatload usb 0:1 0x01080000 uImage

reading uImage
18424384 bytes read in 1697 ms (10.4 MiB/s)
gxl_p212_v1#fatload usb 0:1 $dtb_mem_addr dtb

reading dtb
22401 bytes read in 26 ms (840.8 KiB/s)
gxl_p212_v1#bootm 0x1080000 - $dtb_mem_addr

ee_gate_off ...
## Booting kernel from Legacy Image at 01080000 ...
   Image Name:   linux-next
   Image Type:   AArch64 Linux Kernel Image (uncompressed)
   Data Size:    18424320 Bytes = 17.6 MiB
   Load Address: 01080000
   Entry Point:  01080000
   Verifying Checksum ... OK
load dtb from 0x1000000 ......
      Amlogic multi-dtb tool
      Single dtb detected
## Flattened Device Tree blob at 01000000
   Booting using the fdt blob at 0x1000000
   Loading Kernel Image(COMP_NONE) ... OK
   kernel loaded at 0x01080000, end = 0x02212200
libfdt fdt_path_offset() returned FDT_ERR_NOTFOUND
[rsvmem] fdt get prop fail.
   Loading Device Tree to 000000001fff7000, end 000000001ffff780 ... OK
fdt_instaboot: no instaboot image

Starting kernel ...

uboot time: 86193017 us